Silicon nitride is superior to other ceramics in heat resistance, strength, fracture toughness, and the like, and hence a porous body made thereof is expected to be used as a component for purifying gas and/or solution such as a ceramic filter (for example, a honeycomb structure in a device for purifying an exhaust gas of an internal combustion engine, and the like).
However, silicon nitride is higher in raw material price as compared to other ceramics and furthermore high in sintering temperature so that the production cost thereof inevitably becomes high. Accordingly, although silicon nitride has excellent characteristics as described above, it has not yet come into wide use, as affairs stand.
On the other hand, metallic silicon (Si) is lower in price compared to silicon nitride, and a number of methods have been attempted in which metallic silicon is nitrided by use of a nitriding reaction, that is an exothermic reaction, to obtain a silicon nitride sintered body. As a method for obtaining a silicon nitride sintered body based on such a reaction sintering method, there is disclosed, for example, a method for obtaining a silicon nitride sintered body from a fine Si powder (Japanese Patent Laid-Open No. 52-121613).
In such a reaction sintering method using metallic silicon, generally the sintering is made while nitriding by allowing nitrogen gas to react with a molded body consisting of a mixture composed of a metallic silicon powder and a silicon nitride powder. A silicon nitride sintered body thus obtained is excellent in thermal shock resistance, abrasion resistance, high electric resistance, and chemical stability, and also excellent in that the dimensional change during the reaction sintering is so small that a sintered body having a high degree of dimensional precision can be obtained (during the reaction sintering, the weight increases by about 60%, but the dimensional change is small, so that a relatively fine sintered body can be obtained with almost the same dimension and shape as those of the molded body before the reaction sintering).
However, when such a silicon nitride sintered body is applied to a component for purifying gas and/or solution such as a ceramic filter (for example, a honeycomb structure in a device for purifying an exhaust gas of an internal combustion engine, and the like), in order to improve filter characteristics and purification characteristics, it is necessary to control a sintered body so as to have a porous structure with a high open porosity and a large pore diameter. A nitriding treatment of a metallic silicon molded body having a low molded body density, for a purpose of obtaining a porous body having a high open porosity, yields a porous structure in which fine silicon nitride fibers are generated not only on a surface but also in an interior, to yield a porous structure of a small pore diameter. In a porous body having a small pore diameter, a permeability coefficient becomes small, so that a high pressure is required for permeation of gas or solution. For example, for a solution system, a feed pump capacity needs to be increased.
In addition, when a porous body small in pore diameter is used for an exhaust gas purifying device, a back pressure becomes high to degrade an engine performance. Moreover, when a catalyst is loaded on a porous body, it is possible to obtain a product having a high purification ratio by using a carrier with a large open porosity and a large pore diameter.
The present invention was achieved in consideration of the above described problems, and takes as its object the provision of a silicon nitride porous body, which has a porous structure with a large average pore diameter, from which a test specimen cut out exhibits a large thermal conductivity and a small thermal expansion coefficient, and which is suitably used as a component for purifying gas and/or solution such as a ceramic filter, and an efficient method of manufacturing the same.